At 02:31 PM 5/19/2006, Peter Martinez wrote: > >From G3PLX: > >That's a picture of the broadband noise EMITTED from the audio OUTPUT side >of the soundcard in question. My query is about the digital noise registered >by the input ADC. The tests I have done so far say that there is 8 bits of >background noise (1 bit = 6dB) coming from the 24-bit ADC in the Firebox >with the input shorted. That says there's only 16-bits of useful signal in >there. Why should I pay for 24 bits when 8 of them are below the noise? > >I say again: Is there anybody out there writing their own SDR software that >has actually masked-off the bottom 8 bits or replaced it with random >numbers?
I've done a fair amount of A/D testing for a variety of RF applications, and have a few comments: 1) Have you tried making the noise measurement with a very clean sinewave input at various levels? (this is somewhat non trivial to generate) 2) Is the random low order noise white? Or does it have some spectral shaping? 3) It's possible that the limit of performance is set by something other than preamp noise before the A/D. For instance, if the clock jitter is only good to 16 bits, then there's not much point having a 24 bit noise level in the analog side. 4) It might be that 24 bit cards may not give you 24 bit performance, but might give you 18 bit or 20 bit performance, so you're better off than a 16 bit card giving you 14 bit performance 5) The other thing you get with things like the Firebox is multiple ins and outs. Jim _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
